At its upstream end, a bypass turbomachine has a fan wheel carrying a plurality of blades that rotate inside a fan casing. A cone arranged upstream from the fan wheel serves to direct the incoming air stream through the blades of the fan. In order to prevent air from flowing past the tips of the blades, which would reduce the efficiency of the turbomachine, a coating of abradable material is placed in register with the fan blades and is carried by the radially inner face of the fan casing.
While the turbomachine is operating in flight, ice forms on the surface of the cone and at the radially inner ends of the blade airfoils. Under the effect of centrifugal force, pieces of ice are ejected and then collide with the portion of the fan casing that is situated downstream from the abradable material.
To avoid damaging the inside wall of the fan casing downstream from the abradable material, glass fiber panels supported by damper pins are fastened to the fan casing. Each of these protective panels is made up of a stack of several layers of glass fibers, and the protective panels are connected downstream to soundproofing panels.
Unfortunately, the use of protective panels reduces the axial extent of the soundproofing panels, thereby contributing to increasing the sound level emitted from the turbomachine, even though it is desirable to reduce it. Furthermore, the protective panels need to made as a unit with the soundproofing panels, thereby complicating fabrication thereof and increasing their costs. Finally, during maintenance, replacing a protective panel requires the soundproofing panel that is secured thereto to be replaced as well, thereby significantly increasing costs since soundproofing panels are elements that are difficult to fabricate and therefore expensive.
At present, wide-chord blades, i.e. blades of curved section, are preferred to straight blades since their more highly-developed aerodynamic shape serves to improve the efficiency of the fan, and above all to make it better at withstanding impacts in flight against foreign bodies, such as birds, for example.
Nevertheless, the particular shape of these blades serves to lengthen the ice impact zone upstream into the abradable material that is situated in register with the fan blades. For this type of blade, the impact zone begins about two-thirds of the way along the axial extent of the abradable material and terminates downstream from the blades. Thus, the protective panels commonly used for straight blades are found to be unsuitable for a fan having wide-chord blades when it comes to protecting all of the portion of the casing that is subjected to ice impacts.